1. Field of the Invention
The present invention relates to a high-frequency signal transmission system for use as a microwave antenna, a microwave filter, or the like, which transmits microwave signals with a reduced coupling capacitance in a wide frequency range without distortion and phase delay.
2. Description of the Prior Art
Recent microwave signal transmission such as radio signal transmission for mobile telephone, for example, requires an increase in the range of transmission frequencies and a reduction in the transmission loss.
One known microwave transmission system is disclosed in U.S. Pat. No. 3,909,755 entitled "LOW PASS MICROWAVE FILTER".
FIG. 1 of the accompanying drawings is a perspective view, partly in cross section, of such a conventional low-pass microwave filter. As shown in FIG. 1, the low pass microwave filter has a plurality of cascade-connected conical inner conductors 2a, 2b, 2c, an outer conductor 4 covering the inner conductors 2a-2c, and an insulator tube 6 disposed in close contact between the maximum-diameter outer surfaces of the inner conductors 2a-2c and the inner surface of the outer conductor 4. The insulator tube 6 insulatively holds the inner conductors 2a-2c, and serves as a dielectric member. A connector conductor 8 is joined to the right-hand end (as viewed in FIG. 1) of the inner conductor 2c. A high-frequency signal RFIN is supplied between the connector conductor 8 and an end of the outer conductor 4. Another connector conductor 10 is joined to the left-hand end (as viewed in FIG. 1) of the inner conductor 2a. A high-frequency signal RFOUT is outputted across a load R which is connected between the connector conductor 10 and an opposite end of the outer conductor 4.
Each of the conical inner conductors 2a-2c is a wide-range exponential line. The frequency range of each of the conical inner conductors 2a-2c can be set to a desired range by varying the total length (.lambda./2) and the diameters at the opposite ends thereof. Since the insulator tube 6 which mechanically supports the conical inner conductors 2a-2c serves as a dielectric member, as described above, the frequency range of each of the conical inner conductors 2a-2c is selected in view of the dielectric constant of the insulator tube 6. The high-frequency signal RFIN supplied between the connector conductor 8 and the end of the outer conductor 4 is processed into characteristics corresponding to the transmission characteristics of the high-frequency signal transmission system, and outputted as the high-frequency signal RFOUT between the connector conductor 10 and the opposite end of the outer conductor 4.
The conical inner conductors 2a-2c may be replaced with a plurality of discs having successively greater external dimensions and fixed in position by a shaft extending centrally through the discs. Alternatively, the conical inner conductors 2a-2c and the outer conductor 4 may be switched around in structure. Specifically, the outer conductor 4 may be shaped complementarily to the conical inner conductors 2a-2c, and an insulator member may extend centrally through the outer conductor 4 with a central conductor being disposed in the insulator member. As another alternative, a stripline comprising a plurality of cascaded triangular plates may be used as a substitute for the conical inner conductors 2a-2c.
In the conventional low-pass microwave filter disclosed in U.S. Pat. No. 3,909,755, the insulator tube 6 may be dispensed with, and the inner conductors 2a-2c in the outer conductor 4 may be fixed in place by insulating screws that are made of plastic.
According to the conventional low-pass microwave filter, the inner conductors 2a-2c are positioned in the outer conductor 4 by the insulator tube 6 that is disposed between the inner conductors 2a-2c and the outer conductor 4. Therefore, the low-pass microwave filter develops a great reflected-wave power against the traveling-wave power of a high-frequency signal that is supplied thereto, resulting in a poor standing-wave ratio (V.SWR). More specifically, the dielectric strain of the insulator tube 6 causes a phase delay in the transmitted high-frequency signal, and attachment members develops a loss, thereby failing to generate an isotropic electromagnetic field and hence to provide transmission characteristics equal to the radio wave propagation speed in free space.
The maximum-diameter portions of the cascaded conical inner conductors 2a-2c comprise flat joint surfaces each having a width of .lambda./20 which are held in contact with the insulator tube 6. Therefore, the conical inner conductors 2a-2c are mechanically stably supported in the insulator tube 6. The flat joint surfaces of the conical inner conductors 2a-2c are, however, line portions where the outer configuration of the conical inner conductors 2a-2c is not exponentially represented. Since the coupling capacitance is increased at the flat joint surfaces, it is impossible to construct wide-range exponential lines that are consistent with the theoretical principles. Furthermore, the joints between the conical inner conductors 2a-2c develop a large coupling capacitance due to the dielectric constant of the insulator tube 6, resulting in poor response characteristics which limit the transmission frequency range. Even if the insulator tube 6 is dispensed with and the insulating screws are employed, a parasitic capacitance is produced which results in poor response characteristics which limit the transmission frequency range. Moreover, inasmuch as the opposite ends of the inner conductors 2a-2c and the outer conductor 4 are of a uniform diffraction open structure, the high-frequency signal that is being transmitted leaks as an undesired radiation. Consequently, nearby electronic devices tend to suffer electromagnetic interference (EMI).